The goal of this project is to provide support of National Toxicology Program (NTP) hazard identification activities targeted toward the prevention of diseases or adverse effects caused by environmental exposure to chemical or physical agents. Toxicity testing is an important aspect of public health research in that it serves to identify chemicals that are hazardous to human health. Proper conduct of toxicology studies is required to ensure that the resulting data are reliable. The NTP recently reported the results of toxicity and carcinogenicity studies of radio frequency radiation at frequencies and modulations used by cell phones. Frequencies were 900 MHz in rats and 1900 MHz in mice. Modulations were GSM and CDMA. Exposures were 9 hr 10 min over an 18 hr 20 min period each day (10 min on/10 min off). The goal of these studies is to conduct follow on mechanistic studies on radio frequency radiation. These studies will address questions raised during conduct and reporting of the carcinogenicity studies and extend mechanistic understanding to additional modulations/exposure scenarios. Data from this project will ultimately allow a greater understanding of the toxicity and carcinogenicity studies in rats and mice and support the risk assessment efforts of National Toxicology Program and other federal agencies. Keywords: radio frequency radiation, CDMA, GSM, modulation, frequency